-LRB- CNN -RRB- -- For the first time since a reported massacre there , U.N. observers on Saturday entered the Syrian town of Tremseh , where opposition activists say more than 200 people were killed .

The violence took place Thursday , on what may have been the single deadliest day in the 16-month crisis . It prompted a fresh round of condemnation from world leaders .

In Tremseh , the U.N. team found evidence of an attack , including a burned school , damaged houses , and proof that artillery , mortars and small arms were used , said Sausan Ghosheh , spokeswoman for the head of the U.N. Supervising Mission in Syria . She added that the number of causalities remains unclear .

`` The attack ... appeared targeted at specific groups and houses , mainly of army defectors and activists . There were pools of blood and blood spatters in rooms of several homes together with bullet cases , '' Ghosheh said in a statement .

U.N. observers are expected to return to the town Sunday to continue their fact-finding work .

Still grappling with the attack , Syrians endured yet another bloody day Saturday as regime forces fired from low-flying helicopters and a bomb exploded at a state security headquarters , opposition activists said .

At least 73 people were killed in Saturday violence , including 20 in Homs , 11 in Damascus Suburbs and 13 in Hama province , the Local Coordination Committees of Syria said . Fourteen additional deaths were reported in Deir Ezzor and 12 in Idlib , among others .

Another opposition group , the Syrian Observatory for Human Rights , said a car bombing targeted a state security building in Hama , and `` a number of state security personnel were killed and wounded . ''

Syrian state-run TV said at least three civilians and a security officer were killed in Muhrada by a suicide bomber in a truck .

Farther south , the Daraa province town of Khirbet Ghazaleh came under heavy shelling and machine gun fire after the Syrian army surrounded it with tanks , the LCC said .

`` Helicopters fly over the city at a low altitude with a continued siege of the city and gunfire from snipers , '' the opposition network said .

Meanwhile , Deir Ezzor was subject to intense shelling by government forces , as well as fierce clashes between regime forces and the Free Syrian Army , the LCC said .

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According to the opposition network , more than 200 villagers were killed in the Hama-area town of Tremseh on Thursday , and dozens more were killed elsewhere across the country .

U.N. spokeswoman Ghosheh said a large patrol had been sent from Damascus to Tremseh on Saturday to assess the situation , amid widely differing accounts of what happened from opposition activists and the government .

An initial reconnaissance mission was sent Friday following assurances of a cease-fire in the area , the spokeswoman said , but it was too late in the day to do much .

`` The patrol assessed the situation -- if there was in fact a cease-fire and our access to the town , '' she said . `` An 11 vehicle integrated patrol , comprised of specialized military and civilian observers , arrived ... on Saturday after confirming that a cease-fire was in place . ''

International anger against Syrian President Bashar al-Assad has ratcheted up since the Tremseh incident , with at least one U.S. official suggesting the need for more pressure on al-Assad 's regime .

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`` Through these repeated acts of violence against the Syrian people , President Assad has lost legitimacy to lead . It is time for him to go . It is time for the political transition that is long overdue to finally get under way , '' Josh Earnest , a White House spokesman , told reporters Friday .

`` It certainly does build strong international support ... to continue to ramp up the pressure on Assad , '' he added , citing `` ongoing conversations at the United Nations about additional ways that we can build some international agreement and raise the stakes even further . ''

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Activists in the city of Hama , meanwhile , gave a grisly account of the assault in Tremseh .

Witnesses inside the town told the activists by telephone that Syrian military forces had launched a full-scale attack against the opposition Free Syrian Army inside the town , which was surrounded by government tanks and artillery .

As the government forces rained artillery rounds into the town , a number of village residents fled their houses , going into the streets , where many of them were shot dead by the government militias , the activists told CNN .

The government painted a starkly different picture of Tremseh than that detailed by opposition groups .

The state-run Syrian Arab News Agency blamed `` armed terrorist groups '' for the violence . It said the government said residents called security forces for help after the terrorist groups raided the neighborhood .

Regime forces arrested some of the members of the terrorist groups and confiscated their weapons , the government said .

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`` Armed forces successfully dealt with the terrorists without casualties taking place among the citizens . They searched into the terrorists ' dens where they found the dead bodies of a number of citizens who had been abducted and killed by the terrorist groups , '' SANA reported , citing a military source .

CNN can not independently verify reports from Syria because the nation has restricted access by international journalists .

Meanwhile , many who survive the violence are caught in a precarious humanitarian situation .

The chief U.N. organization that coordinates emergency aid warned Friday that more Syrians will die if contributing nations do not follow through and fund its relief operation .

`` We have run out of language to describe how it is for the civilian population , '' said John Ging , operations director and chairman of the Syria Humanitarian Forum for the U.N. Office for the Coordination of Humanitarian Affairs . `` It is physical and it is psychological . ''

CNN 's Saad Abedine , Dan Lothian and Hamdi Alkhshali contributed to this report .

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NEW : At least 73 people are killed across Syria on Saturday , opposition activists say

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U.N. monitors arrive in Tremseh and find evidence of an attack

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An opposition group says more than 200 people were killed there Thursday

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The Syrian government blames the Tremseh incident on armed terrorist groups